1. Field of the Invention
The present invention relates to an exposure apparatus and a method of manufacturing a device and, more particularly, to an exposure apparatus used in manufacturing a device such as a semiconductor device, an image sensing device, a liquid crystal device, or a thin-film magnetic head by lithography, and a method of manufacturing a device using the same.
2. Description of the Related Art
Recently, in the quest for smaller, thinner electronic devices, demands for advances in micropatterning of semiconductor devices mounted in the electronic devices are further growing. For example, the design rule is about to achieve the formation of a circuit pattern having a minimum feature size of 100 nm or less on mass production lines, and is expected to further shift to the formation of a circuit pattern having a minimum feature size of 80 nm or less in the future. A main stream processing technique for attaining such a strict design rule is photolithography. In photolithography, a projection exposure apparatus which projects and transfers a mask pattern drawn on a mask or reticle onto a wafer using a projection optical system has conventionally been employed.
A resolution R of the projection exposure apparatus is given by the Rayleigh equation:R=k1×λ/NA  (1)where λ is the wavelength of the light source, and NA is the numerical aperture of the projection optical system.
A depth of focus DOF, that is, the focal range within which a constant image forming performance can be maintained is given by:DOF=k2×λ/NA2  (2)
As the DOF decreases, focusing becomes more difficult. To overcome this difficulty, there is a demand to increase the substrate flatness and the focusing accuracy, and so, essentially, the DOF is desirably large.
The mask pattern includes, for example, a line and space (L & S) pattern having adjacent lines and spaces, a contact hole array having adjacent and periodical contact holes (i.e., contact holes arrayed at an interval equal to the hole diameter), an isolated contact hole which is not adjacent to, but isolated, and other isolated patterns. However, to transfer the pattern with a high resolution, it is necessary to select an optimum illumination condition in accordance with the type of pattern.
System chips manufactured by the modern semiconductor industry are shifting to those having a higher added value and a mixture of a wide variety of patterns. Under the circumstances, it has become necessary to fabricate a mask having a mixture of a plurality of types of contact hole patterns. However, it has been impossible to simultaneously transfer contact hole patterns each having a mixture of a contact hole array and an isolated contact hole, with a high resolution.
To combat this situation, various kinds of methods have been proposed for increasing the depth of focus by increasing the resolution limits of only a contact hole array and a two-dimensional repetitive interconnection pattern. An example of these methods is a phase shift method using a double exposure (or multiple exposure) scheme that separately forms, by exposure, different types of patterns using two masks, or a scheme that enhances the resolving power of a main pattern by providing various types of auxiliary patterns to the mask pattern. This method improves the resolving power by forming a thin film which guides the propagating light so that a portion of the conventional mask becomes 180° out of phase with its remaining part.
Unfortunately, various problems remain to be solved in order to improve the resolving power using a phase shift mask of a type that actually modulates the spatial frequency. Because of these problems, it is currently very difficult to manufacture semiconductor devices by actually using the phase shift mask.
A method commonly used at present exposes one mask under a special illumination condition. In contrast to vertical illumination as conventional illumination, this method obliquely applies light onto the reticle by adjusting the effective light source shape to an annular shape or a quadrupole shape, and is called modified illumination (off-axis illumination).
In conventional illumination, an image is formed by interference among three light beams: the 0th- and ±1st-order light beams. In this case, the ±1st-order light beams are distributed at positions, which are shifted by their diffraction angles from the optical axis, in the pupil plane, as shown in FIG. 4A. However, as shown in FIG. 4B, as the pattern becomes finer, the intervals between the 0-th and ±1st-order light beams widen, so a certain component of the diffracted light falls outside of the aperture stop of the projection lens. As a consequence, the 0th-order light beam can interfere with no light beam, and, therefore, no image can be formed.
In modified illumination, an image is formed by interference among two light beams: the 0th- and +1st-order light beams or the 0th- and −1st-order light beams, as shown in FIG. 5B. In this case, an angle θ between the optical axis and the diffracted light in modified illumination is narrower than that in conventional illumination shown in FIG. 5A. Using this fact, the incident angle in off-axis illumination is increased so that the 0th- and +1st-order light beams or the 0th- and −1st-order light beams barely fall within the pupil. With this operation, diffracted light that enters the pupil plane at a larger incidence angle can be received even when the lens NA remains the same. FIG. 6 is a view showing an example of the diffracted light distribution on the pupil plane in annular illumination. Even diffracted light that is shielded by the NA stop in conventional illumination, as shown in FIG. 7A, can be received in the pupil plane in modified illumination, as shown in FIG. 7B. It is, therefore, possible to ensure the contrast of even a fine pattern. That is, it is possible to effectively increase the lens NA. In addition, an angle θ between the optical axis and the diffracted light in modified illumination is narrower than that in conventional illumination, resulting in an increase in the DOF.
Annular illumination is suited to a repetitive dense pattern, so it is effective for general patterns in various directions. In practice, because it is necessary to optimize the annular ratio, as shown in FIGS. 3A and 3B, in accordance with the pattern pitch and direction, a function of continuously changing the outer σ is important.
The light source shape in quadrupole illumination, shown in FIG. 3C, is suited to a two-dimensional repetitive pattern. Quadrupole illumination uses a shape having one opening in each of the four quadrants with respect to the optical axis.
FIG. 8 is a schematic view showing the main portion of a scanning projection exposure apparatus using the conventional 1× mirror optical system (see Japanese Patent Laid-Open No. 2006-019412). The projection exposure apparatus shown in FIG. 8 includes a reflecting projection optical system R including a concave mirror 40, a convex mirror 41, and mirrors 39 and 42. The projection exposure apparatus also includes a mercury lamp power supply 21, an elliptical mirror 22, a shutter 23, condenser lenses 24 and 28, a wavelength filter 25, an integrator 26, a stop 27, a field stop 29, including an arcuate or fan-shaped aperture, and a relay system including a relay lens 30 and mirrors 31 and 32. The projection exposure apparatus also includes an illumination system I that forms an arcuate or fan-shaped illumination region on a mask 38.
The projection exposure apparatus includes a Kohler illumination system arranged such that a secondary light source plane formed by the integrator 26 nearly matches the focal point of the condenser lens 28 on its front side, and that the field stop 29 nearly matches the focal point of the lens 28 on its rear side.
The mask 38 is inserted in the object plane of the projection optical system R, and is driven in synchronism with a wafer 43 inserted in the image plane. The mask 38 and the wafer 43 are scanned by light in directions indicated by a double-headed arrow in FIG. 8, in the object plane and the image plane, respectively, thereby transferring a pattern formed on the mask 38 onto the wafer 43.
The illumination system I is required to uniformly and efficiently irradiate the effective image region (which typically has an arc or fan shape) of the projection optical system R on the mask 38 with a predetermined numerical aperture.
To attain this operation, the conventional illumination system superposes, on the field stop 29, illumination light beams from cylindrical fly-eye lenses used as integrators to temporarily form a rectangular irradiation region free from illuminance nonuniformity on the field stop 29. An image of the light beam that passes through an arcuate or a fan-shaped slit (aperture) formed in the field stop 29 is formed on the mask 38 by the relay system (image forming system), including the optical elements 30 to 32, to obtain illumination with a desired arc or fan shape and an illuminance that is uniform throughout all points in the irradiation region on the mask 38.
Conventionally, a mercury lamp or an excimer laser is used as a light source to illuminate an illumination target object or to expose an exposure target object. These light sources are very inefficient because most of the energy input to drive them turns into heat. Also, assume that projection exposure, in which the feature sizes and arrangements of a pattern having a contact hole array, a pattern having a mixture of an isolated contact hole and a contact hole array, and a mask pattern change for each process is performed on a processing target in, for example, photolithography. In this case, when modified illumination is necessary, exposure is performed by forming necessary illumination after selectively exchanging a stop for modified illumination as needed for a single light source. However, the number and types of stops have limitations, so it is impossible to freely change an illumination condition that exhibits the required function of the illumination optical system (more specifically, the effective light source distribution of the illumination optical system). This makes it impossible to perform exposure under an optimum illumination condition and therefore to obtain a high resolution. In addition, the production efficiency is poor because the stop requires replacement as the illumination condition is changed.
The conventional techniques often have a function of changing a normal circular effective light source to an annular effective light source, or a mechanism that switches to a quadrupole effective light source. However, to keep up with advances in micropatterning in the future, it is necessary to improve the resolving power by flexibly changing effective light source of even the same type.
Especially, in exposure that uses a mercury lamp as a light source, an exposure shutter plays a role of an exposure amount control unit. For this reason, the exposure time is limited by the driving time of such a mechanical shutter, so the throughput rate is controlled by the shutter opening/closing speed. This makes it hard to improve the throughput.